Gas turbines often include a compressor, a number of combustors, and a turbine. Typically, the compressor and the turbine are aligned along a common axis, and the combustors are positioned between the compressor and the turbine in a circular array about the common axis. In operation, the compressor creates a compressed working fluid, such as compressed air, which is supplied to the combustors. A fuel is supplied to the combustor through one or more fuel nozzles and at least a portion of the compressed working fluid and the fuel are mixed to form a combustible fuel-air mixture. The fuel-air mixture is ignited in a combustion zone that is generally downstream from the fuel nozzles, thus creating a rapidly expanding hot gas. The hot gas flows from the combustor into the turbine. The hot gas imparts kinetic energy to multiple stages of rotatable blades that are coupled to a turbine shaft within the turbine, thus rotating the turbine shaft and producing work.
To increase turbine efficiency, modern combustors are operated at high temperatures which generate high thermal stresses on various components disposed within the combustor. As a result, at least a portion of the compressed working supplied to the combustor may be used to cool the various components. For example, many modern combustors may include a generally annular cap assembly that at least partially surrounds the one or more fuel nozzles. The cap assembly may generally provide structural support for the one or more fuel nozzles, and may at least partially define a flow path for the fuel-air mixture to follow just prior to entering the combustion zone. Certain cap assembly designs may include a generally annular cap plate that is disposed at a downstream end of the cap assembly and that is adjacent to the combustion zone. As a result, the cap plate is generally exposed to extremely high temperatures, thus resulting in high thermal stresses on the cap plate. In addition, high combustion dynamics resulting from pressure oscillations within the combustion zone may combine with the high thermal stresses to significantly limit the mechanical life of the cap plate.
Current cap assembly designs attempt to mitigate the high thermal stresses by directing a portion of the compressed working fluid to the cap assembly and through multiple cooling holes which extend through the cap plate surface. This method is known in the industry as effusion cooling. However, the compressed working fluid flowing through the multiple cooling holes may enter the combustion zone generally unmixed with the fuel. As a result, NOx and/or CO2 generation may be exacerbated and turbine efficiency may be decreased. Therefore, a combustor that provides cooling to the cap assembly and improves pre-mixing of the compressed working fluid with the fuel for combustion would be useful.